This invention relates to a process for purifying alumina from low-grade alumina-bearing ores, e.g., such as gibbsitic bauxite containing high concentrations of kaolinite. In one aspect, this invention relates to a process combining physical pretreatment and chemical extraction to purify alumina from low-grade bauxite ores.
Low-grade bauxite ores, e.g., such as bauxites containing high concentrations of reactive silica cannot be processed economically by the conventional Bayer process for the reason that high caustic soda losses are incurred by precipitation in the residue. By "high reactive silica content" is meant more than about 5-6% by weight reactive silica in the ore. Conventionally, high reactive silica bauxites are processed domestically by a combination process including a Bayer process of pressurized digestion at 143.degree. C. for 0.5 hr. with the digest residue (red mud) processed by a lime-soda sinter to recover the Na.sub.2 O and Al.sub.2 O.sub.3 fixed in the residue as desilication product (DSP).
Physical beneficiation methods have been investigated previously to lower the silica content of high reactive silica bauxites. However, particle size separation by washing or particle size separation followed by flotation of the fines fraction have been unsuccessful in producing an acceptable Bayer plant feed, because of low available Al.sub.2 O.sub.3 recovery and/or high reactive silica in the product.
A particle size pretreatment step was described in Fish, U.S. Pat. No. 3,681,013, to provide a process for caustic digestion of bauxite while eliminating non-reactive silica problems encountered in a high-temperature, high-pressure digestion. The Fish process involves separating a coarse fraction from a fines fraction of bauxite prior to digesting the fines according to a conventional high-temperature and high-pressure Bayer process system. Fish discloses that this may be done by a dry or a wet process. Fish discloses the fines fraction can be separated from the coarse fraction while both are dry, or the coarse fraction may be separated from the fines while digesting the bauxite with spent liquor, generally supplemented by additional caustic, at substantially atmospheric pressure to extract alumina from the coarse fraction. Spent liquor is the liquor remaining after precipitation of alumina following the high-temperature, high-pressure digestion. Liquor entering the conventional high-temperature and high-pressure digestion stage is rich in alumina removed from the separated coarse fraction. Fines are introduced to the high-temperature, high-pressure digestion stage either separately or suspended in the liquor which contains alumina extracted from the coarse fraction. In the Fish process, the spent liquor preferably does not contact the fines when the separation of coarse from fines is accomplished as a part of the preliminary low-pressure digestion stage.
The Fish process proposed the particle size separation step after alumina is extracted from the coarse fraction by introducing spent liquor at the opposite end from which the bauxite is introduced to one or more vessels in which the preliminary digestion takes place at substantially atmospheric pressure. The Fish process preferably is operated at a temperature no higher than the atmospheric boiling point, e.g., such as at a temperature in the preliminary digestion stage ranging from about 170.degree. F. to about 230.degree. F.
The Fish patent describes the purification of bauxite containing approximately 40% total Al.sub.2 O.sub.3 and 28% total SiO.sub.2 (only approximately 0.8% reactive SiO.sub.2) by preliminary digestion with caustic supplied countercurrently as caustic spent liquor having an alumina to total caustic (expressed as grams per liter sodium carbonate) ratio of about 0.33 from a conventional Bayer precipitation step at substantially atmospheric pressure and a temperature of about 200.degree. F. Extraction from the coarse fraction was substantially complete as confirmed by an analysis of the coarse mud. An overflow stream containing fines fraction (-40 mesh) and alumina removed from the coarse fraction at a silica to total caustic ratio of 0.0047 and an alumina to total caustic ratio expressed as grams per liter sodium carbonate of about 0.48. Digestion was continued under conventional high-temperature, high-pressure conditions at a temperature of 290.degree. F. and a pressure at 43 psig until the alumina to total caustic ratio was about 0.63. The resulting sodium aluminate liquor was then flashed back to substantially atmospheric pressure and temperature prior to having the alumina separated therefrom by precipitation by seeding.
Atmospheric pressure digestion of bauxite slurry with caustic at a temperature at or below the boiling point has been recognized as a process for solving the silica problem in low-grade bauxite ores. Such an atmospheric pressure digestion was recognized as prior art in the Background of the Invention in the aforementioned Fish patent. Fish also recognized a prior art process to reduce silica problems by first digesting the bauxite with a sufficient concentration of caustic at a sufficient temperature and pressure to dissolve much of the silica and form a desilication product without dissolving any appreciable amount of alumina and then, without separating the resulting dispersed solids from the resulting liquor, continuing the digestion at the conventional high temperatures and pressures. The difficulty with this procedure is that coarse particles of undigested bauxite or of silica in the form of desilication product will still be present in the unseparated solids. Such a procedure is illustrated in Roberts, U.S. Pat. No. 3,413,087.
The Roberts process involves contacting a thick slurry of fresh comminuted ore with caustic alkali solution, the amount of the solution being insufficient to dissolve all of the soluble alumina but sufficient to dissolve substantially all the silica in the ore, and the slurry is treated to precipitate substantially all the silica present. Then the ore slurry is heated by indirect heat exchange with recovered steam, and the heated slurry is passed to the digestion stage, while the resulting spent liquor containing residual silica is partly recycled to the initial ore preparation stage.
Another prior art process involving a low-pressure, low-temperature digest, e.g., such as at room temperature and atmospheric pressure, is practiced in Africa at Fria, Guinea, with low SiO.sub.2 bauxite. The Fria plant operates at a high concentration of caustic, i.e., slurry discharged from rod mills flows into a tank where it is mixed with additional sodium aluminate liquor at 200 grams per liter Na.sub.2 O. However, such a system involves a considerable amount of dilution in the subsequent alumina hydrate precipitation stage, necessitating a complex evaporation unit if a concentrated caustic is to be reused.
The aforementioned Fish patent, U.S. Pat. No. 3,681,013, recognizes another prior art process involving grinding to reduce silica abrasion problems in which the bauxite is ground to such a fine extent that there is no coarse fraction high in silica. The Fish patent reports that this process has tended to increase rather than decrease difficulties, however, as grinding energy requirements have been greater, clarification of fine mud has required additional equipment, washing has been harder, and there has been additional soda loss.
Various other prior art processes have been proposed for the commercial extraction of alumina from high-silica, alumina-bearing ores involving alternative chemical beneficiation steps. Brown, in U.S. Pat. Nos. 2,375,342 and 2,375,343, proposed a method for recovering alumina from low-grade ores by treating the low-grade ore to solubilize its alumina and to separate dissolved alumina from silica and other unwanted impurities in the ore. Alkaline earth and alkali metal compounds are mixed with ore and then sintered. The function of the alkaline earth compound is to insolubilize silica. Any compound which forms insoluble silicates can be used in the Brown process, but readily available and low-cost limestone is preferred. The amount of limestone to be added depends on the amount of silica in the ore. The sintered mixture is then leached to recover soluble alumina and caustic values. The leach liquor will also contain quantities of solubilized impurities, principally silica. The leaching media may be heated, preferably to temperatures not in excess of 200.degree. F., higher temperatures promoting the formation of insoluble complexes containing alumina. The residue of alkaline earth silicate and insoluble impurities plus alumina is discarded. Leach liquor containing substantial amounts of soluble silica is added to the bauxite-caustic liquor and digested in a conventional Bayer process.
Misra, U.S. Pat. No. 4,468,375, notes that caustic soda reacts with silica minerals present in bauxite, and terms this a "chemical caustic loss" dependent upon the amount of caustic and reactive silica minerals present in the bauxite. Misra notes that the Bayer process alumina product generally contains various inorganic impurities such as silica, and the process efficiency is lowered by such impurities which accumulate in the caustic liquor as it recirculates through the initial step of bauxite digestion in the Bayer process. Misra proposed a process including comminuting aluminum-containing mineral ore; reacting the comminuted ore at an elevated temperature with an aqueous solution of at least 150 grams per liter of sodium bicarbonate to form a solid reaction product of dawsonite and impurities; and converting the dawsonite to alumina. In this way, an aqueous solution of sodium bicarbonate replaces aqueous sodium hydroxide in the initial bauxite digestion.
Oku et al., U.S. Pat. No. 3,716,617, discloses a process for producing alumina according to the Bayer process and separating digestion residue from sodium aluminate slurry. Although ambiguous, Oku refers to a "reactive silica" as silica present as clay and/or any other silicate in an alumina-containing ore. Oku discloses that when the temperature during extraction of an alumina component from bauxite is high, digestion time required may be short but, on the other hand, the rate of dissolution of the reactive silica becomes greater, so the rate of variation of the reactive silica content in the digestion residue becomes quick and the operation becomes difficult. Further, the desilication reaction is accelerated and the alumina and alkali solution are lost. On the other hand, if the digestion temperature is low, the desired high alumina concentration in the sodium aluminate solution cannot be obtained. Therefore, the digestion temperature is usually 90.degree. C. to 150.degree. C., preferably 110.degree. C. to 140.degree. C. The Oku process mixes bauxite containing over about 10% by weight total silica, 8.5% by weight reactive silica, with sodium aluminate solution which apparently is silica-free. The digestion residue is separated from the sodium aluminate solution by a synthetic organic high molecular weight flocculent, the separation being conducted only when at least 5% by weight reactive silica remains in the digestion residue. Soda concentrations higher than 70 grams per liter are disclosed for digestion.
It is an object of the present invention to produce alumina from gibbsite-rich ore containing high silica content.
It is another object of the present invention to recover alumina from low-grade bauxite ores by alumina purification while inhibiting the dissolution of reactive silica in the digestion process.
It is yet another object of the present invention to purify alumina-rich ore containing high amounts of reactive silica while minimizing the loss of soda in the form of desilication product.
These and other objects will become apparent from the disclosure which continues as follows.